Glass laminates can be used as windows and glazing in architectural and vehicle or transportation applications, including automobiles, rolling stock, locomotive and airplanes. Glass laminates can also be used as glass panels in balustrades and stairs, and as decorative panels or coverings for walls, columns, elevator cabs, kitchen appliances and other applications. As used herein, a glazing or a laminated glass structure can be a transparent, semi-transparent, translucent or opaque part of a window, panel, wall, enclosure, sign or other structure. Common types of glazing that are used in architectural and/or vehicular applications include clear and tinted laminated glass structures.
Conventional automotive glazing constructions include two plies of 2 mm soda lime glass with a polyvinyl butyral (PVB) interlayer. These laminate constructions have certain advantages, including low cost and a sufficient impact resistance for automotive and other applications. However, because of their limited impact resistance and higher weight, these laminates exhibit poor performance characteristics, including a higher probability of breakage when struck by roadside debris, vandals and other objects of impact as well as well as lower fuel efficiencies for a respective vehicle.
In applications where strength is important (such as the above automotive application), the strength of conventional glass can be enhanced by several methods, including coatings, thermal tempering, and chemical strengthening (ion exchange). Thermal tempering is conventionally employed in such applications with thick, monolithic glass sheets, and has the advantage of creating a thick compressive layer through the glass surface, typically 20 to 25% of the overall glass thickness. The magnitude of the compressive stress is relatively low, however, typically less than 100 MPa. Furthermore, thermal tempering becomes increasingly ineffective for relatively thin glass, e.g., less than about 2 mm.
In contrast, ion exchange (IX) techniques can produce high levels of compressive stress in the treated glass, as high as about 1000 MPa at the surface, and is suitable for very thin glass. Ion exchange techniques, however, can be limited to relatively shallow compressive layers, typically on the order of tens of micrometers. This high compressive stress can result in very high blunt impact resistance, which might not pass particular safety standards for automotive applications, such as the ECE (UN Economic Commission for Europe) R43 Head Form Impact Test, where glass is required to break at a certain impact load to prevent injury. Conventional research and development efforts have been focused on controlled or preferential breakage of vehicular laminates at the expense of the impact resistance thereof.
For certain automobile glazings or laminates, e.g., windshields and the like, the materials employed therein must pass a number of safety criteria, such as the ECE R43 Head Form Impact Test. If a product does not break under the defined conditions of the test, the product would not be acceptable for safety reasons. This is one reason why windshields are conventionally made of laminated annealed glass rather than tempered glass.
Tempered glass (both thermally tempered and chemically tempered) has the advantage of being more resistant to breakage which can be desirable to enhance the reliability of laminated automobile glazing. In particular, thin, chemically-tempered glass can be desirable for use in making strong, lighter-weight auto glazing. Conventional laminated glass made with such tempered glass, however, does not meet the head-impact safety requirements. One method of forming a thin, chemically-tempered glass compliant with head-impact safety requirements can be to perform a thermal annealing process after the glass is chemically-tempered. This has the effect of reducing compressive stress of the glass thereby reducing the stress required to cause the glass to break. A disadvantage of this method is the reduction of compressive stress occurs in all areas of the glass product rather than in the area of the glass where the head impact is most likely to occur.
Thus, there is a need to perform localized annealing in controlled areas of the glass whereby a resulting product would retain its strength in critical areas, e.g., near the edges thereof, and be weakened in the areas important to occupant safety.